1. Field of the Invention
The present invention relates to a color filter applicable to a system for displaying a picture or imaging(recording) a picture or other picture Processing systems and a color picture display device using the same. Particularly, the present invention relates to an improvement of a total light utilization factor in a reflection type color picture display device.
2. Description of the Prior Art
Recently, a projector for displaying a picture on a large screen of a display device, such as an outdoor public display, a management purpose display or a display for displaying a high definition video image has been highly required
The projector can be generally classified to a transmission type and a reflection type. In both types, a construction in which read-out light is directed to an LCD (Liquid Crystal Display) panel and modulated in pixel unit correspondingly to a video signal is employed.
The LCD panel is constructed with switching elements such as thin film transistors formed on a semiconductor substrate, an active matrix substrate on which pixel electrodes whose potentials are controlled by the switching elements are arranged, a common electrode film formed on a transparent substrate (glass substrate etc.) and a liquid crystal layer sealed in between the active matrix substrate and the common electrode film.
Read-out light is modulated by making potential differences between the common electrode film and the respective pixel electrodes different correspondingly to the video signal to control orientation of the liquid crystal.
The difference between the transmission type projector and the reflection type projector resides in that, in the former type projector, the active matrix substrate is transparent and light transmitted through the LCD panel is used as projection light, while, in the latter type projector, the pixel electrodes on the active matrix substrate are constructed as electrodes for controlling orientation of liquid crystal through reflection electrodes or dielectric mirror films, etc., and reflection light reflected by the LCD panel is used as projection light.
In general, comparing with the transmission type, the reflection type projector does not require black stripes on the liquid crystal layer and an aperture factor of the liquid crystal cell portion is large.
Further, since, in the reflection type projector, heat generation due to absorption of read-out light is very small, it is possible to direct read-out light having large power to the LCD panel, resulting in brighter picture.
In a conventional color picture projector of transmission type, a color picture is obtained by using three transmission type LCD panels corresponding to three primary colors (R, G, B) and a three color synthesizing optical system for synthesizing lights transmitted through the LCD panels.
Since, however, such projector becomes large in size and its manufacturing cost becomes high, a device in which a single color filter including transparent pixel electrodes for the respective colors of the LCD panel provided in a stripe, mosaic, or delta arrangement and filter elements of the respective colors arranged correspondingly to the pixel electrode arrangement and by which a color projection light can be obtained by using a single system has been proposed.
Incidentally, the filter elements of the color filter for the respective primary colors are adjacently located in different positions on a flat plane. These filter elements located in the different positions constitute a sub pixel having a small area and lights from the adjacent R, G, and B filter elements are sensed by eyes as a pixel having a color which is a mixture of R, G, and B colors.
In the projector having the described construction, however, only one primary color of the read light (white light) which passed through the LCD panel and is incident on the color filter can pass through the color filter and color components of the other 2 primary colors thereof are not utilized.
Further, since the transmittance of the color filter itself is low and, in addition, the transmission type LCD panel has stripes, a total utilization factor of light of such projector is very low.
In view of this, a color filter utilizing a transmission type hologram has been proposed for the color picture projector of the transmission type (cf. Japanese Patent Application Laid-open Nos. H2-500937 and H6-308332).
FIG. 12 shows an example of the color picture projector disclosed in Japanese Patent Application Laid-open No. H6-308332. In FIG. 12, a color filter 52 of a transmission type hologram is arranged opposed to an LCD panel 51. The color filter 52 diffracts and spectroscopically separates an incident read light to respective R, G, and B color components by the diffraction and spectroscopic separation functions thereof and condenses them to transparent pixel electrodes 51r, 51g, and 51b of the LCD panel 52 corresponding to the respective colors.
The transparent type hologram of the color filter includes an array of unit holograms 52p arranged with the same pitch as that of groups of the transparent pixel electrodes 51r, 51g, and 51b on the side of the LCD panel 51. The unit hologram 52p condenses wavelength band components of R, G, and B colors to the respective transparent pixel electrodes 51r, 51g, and 51b while making the angle of diffraction of these components different.
Therefore, according to this construction, it is possible to realize a color projector which can utilize the incident light efficiently.
On the other hand, Japanese Patent Application Laid-open No. H2-500937 discloses, in addition to a similar transmission type projector to that mentioned above, which uses a color filter having a transmission type hologram constituted with holography lens arrays, a reflection type projector.
FIG. 13 shows an example of the transparent type projector.
In FIG. 13, reference numerals 61, 62, and 63 denote a color filter having laminated three holography lens array layers, a glass substrate and a transparent type LCD panel, respectively.
The LCD panel 63 is constituted with a transparent common electrode 64, a liquid crystal layer 65, a reflection film 66 and a pixel electrode layer 67 having R, G, and B pixel electrodes 67r, 67g, and 67b arranged thereon, all of which are laminated. The respective pixel electrodes 67r, 67g, and 67b are supplied with potentials by electron beam or control light beam scanning a rear surface of the LCD panel 63.
The color filter 61 is constituted with a holography lens array 61r for diffracting only R color component of a read light, a holography lens array 61g for diffracting only G color component and a holography lens array 61b for diffraction only B color component, all of which are laminated. As shown in FIG. 13, the respective holography lenses 61r, 61g, and 61b are arranged with a pitch three times of the pitch of the pixel electrodes 67r, 67g, and 67b.
In this projector, the virtual lenses, that is, the holography lens arrays 61r, 61g, and 61b, diffract only color components related to the respective arrays and condense them onto the pixel electrodes 67r, 67g, and 67b arranged on optical axes of the respective lenses.
Although areas covered by the respective lenses are overlapped, the array 61r diffraction only the R color component and passes the G and B color components since the lens arrays 61r, 61g and 61b diffract only the corresponding wavelength band components of the read light, respectively. Similarly, the lens array 61g diffracts only the G color component and passes the B color component and the lens array 61b diffraction only the B color component.
As a result, the R, G, and B color component lights diffracted by the respective holography lens arrays 61r, 61g, and 61b are incident on the liquid crystal layer 65, reflected by the reflection film 66 corresponding to the areas of the pixel electrodes 67r, 67g, and 67b and then incident again on the lens arrays 61r, 61g, and 61b.
The reflected R, G, and B color component lights are modulated in the liquid crystal layer 65 in pixel unit.
The modulated respective color component lights are incident again on the lens arrays 61r, 61g, and 61b, diffracted again thereby and returned in a direction toward a light source of the read light.
In general, in order to improve the diffraction efficiency of hologram (a ratio of intensity of a primary diffraction light to intensity of incident, regenerative illumination light), it is necessary to make an angle between a reference light and objective light in producing hologram large.
In view of this, the angle (.theta.) between reference light and objective light is made large so that the read light falls on the color filter 61 at the incident angle .theta., in this projector.
Therefore, it is necessary, in this projector, to direct the read light to the color filter 61 through a polarizing beam splitter (not shown) and to project a projection light through the same. However, when such polarizing beam splitter is used, contrast ratio is substantially lowered and light utilization factor is lowered due to the angle dependency thereof
Further, since the polarizing beam splitter itself is expensive, the cost of the whole projector becomes high.
3. Description of Previously Proposed Art
Under the circumstance, the present inventors have studied the diffraction efficiency characteristics of hologram in detail. As a result, the present inventors have found that a P polarized light component of diffraction light diffracted by hologram (a polarized light component having a vibration plane parallel to an incident plane) is different in diffraction efficiency from an S polarized light component (a component vibrating in a vertical direction to the P polarized light component) and have proposed a reflection type color picture projector and a color filter for use therein which has a high light utilization factor and generally does not require a beam splitter and with which a high contrast ratio is realized (Japanese Patent Application No. H7-315956).
The proposed projector is based on the characteristics that, when the bend angle (an angle between an incident light and a diffraction light) of the diffraction phenomenon due to hologram is large, there is no substantial difference in diffraction efficiency between the S polarized light component and the P polarized light component, with the difference being increased with reduction of the bend angle.
This characteristics will be described in detail with reference to FIG. 14.
A characteristics curve shown in FIG. 14 was obtained by calculating diffraction efficiency of P polarized light component when a thickness t of hologram is set such that, under condition of incident light wavelength of 540 nm and a modulation amount .DELTA.n of diffractivity for a hologram sensitive material of 0.03, diffraction efficiency of S polarized light component at respective bend angles becomes 100%.
As is clear from FIG. 14, with large bend angles, diffraction efficiency of both the S and P polarized light components is substantially 100%. With bend angle equal to or smaller than 120 degrees, it is possible to make diffraction efficiency of the P polarized light component equal to or smaller than 50% and, by making bend angle closer to 90 degrees, the diffraction efficiency can be made 0%.
Further, this diffraction efficiency characteristics substantially depends upon the incident light wavelength. Therefore, by utilizing this wavelength dependency of the diffraction efficiency characteristics, it is possible to perform an optimum design of the color filter such that the S polarized light component is diffracted at diffraction efficiency of substantially 100% while the diffraction efficiency of the P polarized light component is very small, for a desired wavelength.
Therefore, it is possible to constitute a color filter utilizing transmission type hologram as a holography lens array which diffracts only the S polarized light component in wavelength bands of the respective R, G, and B colors at high diffraction efficiency while restricts the diffraction efficiency of the P polarized light component.
FIGS. 15 to 17 show relations between diffraction efficiency and incident light wavelength of holograms for R, G, and B color lights based on the optimized design conditions when bend angle is 75 degrees, respectively.
In FIGS. 15 to 17, solid curves indicate S polarized light component and dotted curves indicate P polarized light component. From FIGS. 15 to 17, it is clear that diffraction efficiency of S polarized light component of each of the R, G, and B color components is about 100% in the vicinity of a center wavelength thereof and that of P polarized light component at the same wavelength is restricted to equal to or smaller than about 18%.
When the color filter constituted with holograms having the characteristics shown in FIGS. 15 to 17 is used in a reflection type color picture projector with an incident angle .theta. of a read light on the color filter being 75 degrees (=180-105; bend angle=105 degrees), it is possible that the holograms for the respective colors mainly diffract only S polarized light components and emit S polarized light components vertically to the side of the pixel electrodes for the corresponding colors.
The S polarized light components emitted from the color filter and incident on a surface of the liquid crystal layer are reflected by a pixel electrode side of the liquid crystal layer and incident again on the color filter through the liquid crystal layer. In this passage, the S polarized light components are modulated.
For example, when the liquid crystal layer includes vertically oriented liquid crystal, the state of orientation of the liquid crystal is changed correspondingly to a video signal supplied to the pixel electrodes for the R, G, and B color components and the S polarized light components are modulated correspondingly to the video signal. Thus, a portion or whole of the S polarized light component becomes P polarized light component depending upon the degree of modulation and is incident again on the color filter.
The P polarized light component incident again on the color filter passes therethrough without substantial diffraction and is emitted therefrom since he color filter mainly diffracts only the S polarized light component.
The main feature of the projector proposed in Japanese Patent Application No. H7-315956 by the same assignee as that of this application resides in that the P polarized light component which is incident again on the color filter, passes therethrough and emitted therefrom is used as a projection light to project a color picture on a screen through a projection lens, etc.
Therefore, the projection light is the P polarized component which does not return to the direction toward the light source of the read light and is emitted in a normal direction of the color filter. As a result, it is possible to display a high quality color image having a good contrast ratio with high utilization factor of light and without necessity of providing a polarization beam splitter.
The P polarized light component of the read-out light which is incident on the color filter passes straight therethrough as zero order light since it is not diffracted substantially. Since this P polarized component falls on the color filter at the incident angle of 75 degrees, it is reflected by the pixel electrode side surface of the color filter at a reflection angle equal to the incident angle and then incident again on the color filter. Looking this phenomenon from the color filter, the incident angle of the P polarized component on the color filter is -75 degrees. Therefore, it passes therethrough as it is without diffraction.
Therefore, the P polarized component which becomes the zero order light is emitted in a direction which is completely different from the incident direction of the S polarized light component which becomes the projection light or the read light, so that it does not give any influence on the picture display.
Although the S polarized component produced correspondingly to the degree of modulation in the liquid crystal layer and the S polarized component which is produced by modulation of the P polarized component which is diffracted by the color filter with low diffraction efficiency are also incident on the color filter. These S polarized components, however, return toward the light source of read-out light.
As mentioned previously, the present inventors have noticed the fact that the diffraction efficiency of P polarized light component of a hologram for an incident light is much different from that of S polarized light component under the constant condition and proposed in Japanese Patent Application No. H7-315956 a reflection type color picture projector using a color filter constructed with transmission type holograms in which a read light is diffracted and polarized to wavelength bands of respective R, G and B colors, only S polarized light components are mainly diffracted and emitted to the side of pixel electrodes related to the respective colors corresponding thereto and only P polarized light component obtained by modulating the S polarized light component by a liquid crystal layer is emitted from the color filter as a projection light and the color filter itself.
In the proposed color filter, in order to increase the difference in diffraction efficiency between the S polarized light component and the P polarized light component, the incident angle .theta. of the read light with respect to the color filter is set equal to or larger than 60 degrees and smaller than 90 degrees, preferably, in a range from 70 to 80 degrees.
However, when the incident angle of the read light is set to such large value, a cross sectional area of the read light taken vertically with respect to a propagating direction of the read light becomes small, resulting in a reduction of illumination efficiency. That is, there is a relation Sr=Sa.times.cos .theta. between a cross sectional area Sr of the read-out light, an illuminated area Sa for the color filter and the incident angle .theta.. Therefore, when the incident angle .theta. is increased, Sr becomes very small since Sa is constant and, when Sr becomes small, the illumination efficiency is reduced for the reason mentioned below.
In general, although, in order to improve the contrast ratio and the color reproducibility characteristics, it is preferable to make the read light as parallel as possible, it is impossible to obtain a completely parallel light since a light source has a definite size.
Therefore, it is impossible to efficiently collimate the read light to a small area such as a color filter used in a color picture projector. In view of this, it is preferable in order to increase the utilization factor of illumination light to make the cross sectional area Sr of the read light as large as possible.
In other words, the fundamental condition of a color filter that, in order to diffract only S polarized light component attributing to the projection light, the incident angle .theta. is increased by making diffraction efficiency of S polarized light component as large as possible while making diffraction efficiency of P polarized light component as small as possible and the condition for increasing the utilization factor of the illumination light are contradict each other.
In view of the above, the present inventors have further studied the diffraction efficiency characteristics of P polarized light component and S polarized light component due to holograms and found that a color filter which is superior than the color filter disclosed in Japanese Patent application No. H7-315956 can be constructed.